242 PROCEEDINGS OF THE ACADEMY OF [1893. 



abruptly to the aperture, while the back of the whorl keeps straight, 

 horizontal (Man. Conch., PI. 32, fig. 25) or even a little ascending, to 

 the very margin, as we see it in V. parvida, and in some forms of 

 V. costata, while in most of them, (3) as a rule, the whole whorl 

 descends slightly (Man. Conch., PI. 32, fig. 21). 4. Still others have 

 the last whorl near the aperture markedly and decidedly entirely des- 

 cending, so as to form a distinct curvature of the back as in V. 

 perspectiva (Man. Conch. PI. 33, fig. 42), cyclophorella (Man. Conch., 

 PI. 33, fig. 37) and others. The suture either descends comparatively 

 more than the other part, or not so decidedly, as in the Kroellwitz 

 V. tenidlabris (?) These formations have a marked influence on the 

 shape and direction of the aperture, for the latter will necessarily 

 be more inclined where the last whorl is strongly decurved. Where 

 only or principally the suture descends, the aperture will be more 

 circular, and less crescentoid, as becomes apparent at once if we 

 compare V. pulchella and excentrica with parvula and costata. 



In different forms the last whorl not only descends to the aper- 

 ture, but previously to that rises more or less gradually to or above 

 the niveau of the penultimate whorl (Man. Couch., PI. 33, fig. 33, 

 37), thus causing the spire to appear somewhat papilliform, if the 

 shell is viewed from the front. This we find most marked in V. 

 cyclophorella, tenuilabris (Man. Conch., PI. 33, fig. 32), and others in 

 which the penultimate whorl appears narrowed above (Man. Conch., 

 PI. 33, fig. 30). This ascending is directly associated with rapid 

 recession outward as seen in those species at the base. The same 

 thing, though less strongly marked, is found also in many speci- 

 mens of V. costata, and, not infrequently, in pulchella. We have in 

 Helix (Pedinogyra) cunninghami Gray, 10 a very illustrative model 

 of the described configuration of the last whorl, especially on the 

 upper side. 



The aperture is described as being more or less oblique. We 

 must here distinguish its direction as compared with the vertical 

 axis of the shell, which may be called its inclination, from its direc- 

 tion compared with the horizontal axis, or its obliquity. The latter is 

 very marked in V. parvida and perspectiva (Man. Conch., PI. 33. fig. 

 42) and others, where a prolonged horizontal line through the aper- 

 ture would just touch the circumference of the shell, that is, would 

 be tangential, or cut off a very small part of it. We find the con- 



10 In Tryon-I'ilsbry, Man. of Conch., (2) VI, pi. 2, fig. 30, it is well shown, 

 although not especially mentioned in the text (p. 12). 



